Frequently, in a variety of downhole operations, portions of the wellbore need to be isolated. Regardless, of the procedure going on at the time, be it drilling, completion or workover, the tool frequently employed is a packer or bridge plug, which may or may not be retrievable. Frequently, the sealing element is one or more long cylindrical elastomeric members mounted over a mandrel. Setting involves longitudinal compression of the sealing element, with provisions at the ends to prevent extrusion. Longitudinal compression reduces the overall length of the sealing elements and increases their diameter. Frequently, to hold differential forces in excess of thousands of pounds, the sealing element assembly could be set with applied forces of 16,000 or more.
A close examination of the shape changes undergone by the initially cylindrical sealing elements reveals that a twisting effect occurs. It can take the form of a single helical external groove as the compressive load initiates a twisting movement. It can also take the form of opposing exterior helical grooves to the twist imparted to the elements as they are longitudinally compressed.
This buckling phenomenon is illustrated in FIGS. 1 and 2 for the prior designs. In FIG. 1, the sealing element 10 is shown in part in the run in condition where it has a generally cylindrical shape around a mandrel 12. As a result of longitudinal compression, the element 10 takes a spiral shape with a series of points labeled point A moving away from mandrel 12, while at the same elevation but 180 degrees around the outer surface 14, point B moves toward the mandrel 10. Although a single helical pattern 16 is shown in a rather open helix, as a result of the high setting forces applied, the actual appearance of the pattern of helical groove or grooves 16 is more closely akin to elongated narrow void areas in close contact with the casing 18, as shown in FIG. 3.
The system of peripheral grooves 16 is problematic in that it represents potential helical leak paths around the outside of the element 10 regardless of the amount of applied longitudinal compression. Although this phenomenon is a distinct disadvantage, prior designs have configures the sealing element to deliberately undergo such helical collapse pattern under longitudinal pressure on the theory that sealing performance is improved. In U.S. Pat. No. 6,318,461 disc shaped components are used for the sealing element to promote the exterior helical recessed areas but no recognition is given as to the detrimental effects. FIG. 9 of that patent illustrates the exterior spiral present after compression. This reference shows that those working in the field have heretofore had no appreciation that the tendency of elongated cylindrical shapes to twist as they collapse from longitudinal loading could present a situation degrading the desired seal after high expansion of the elements. The apparatus of the present invention recognizes this problem and deals with it in a simple and effective manner. The nature of the solution will be appreciated by those skilled in the art from a review of the description of the preferred embodiment and the claims, which appear below.